A balance-to-unbalance transformer is one of the most commonly used passive elements in wireless communication systems. A balun is a device for converting signals between an unbalanced circuit structure and a balanced circuit structure. The signal of a balanced circuit structure comprises two signal components, called balanced differential signals, which are with same magnitude but 180-degree phase difference. The transmission of differential signals can be used to reduce the common mode noise interference. Therefore, a balun is usually designed in a part of a radio frequency (RF) transceiver, power amplifier (PA), antenna and mixer circuit.
There are several types of baluns, including lumped-type (lattice type), coil-type and distributed-type baluns. A lumped-type balun uses lumped capacitors and inductors to match impedance and generate two balanced signals with same magnitude and 180-degree phase difference. The advantages of a lumped-type balun are small volume and light weight. However, it is not easy to maintain the 180-degree phase difference and the identical magnitude between the two signals.
Coil-type baluns are widely used in lower frequency and ultra high frequency (UHF) bands. When a coil-type balun is used in higher than the UHF band, it usually has a drawback of having considerable loss. In addition, it has reached the limit of miniaturization and can not be further reduced in size.
Distributed-type baluns can further be classified as 180-degree hybrid and Marchand. A 180-degree hybrid balun has a fairly good frequency response in the microwave frequency band. However, its size often poses a problem when it is used in the radio frequency range between 200 MHz and several GHz. Because a 180-degree hybrid balun comprises a few sections of quarter wave transmission lines, it is difficult to reduce the size. Even if it is manufactured in a meandered way, a significant area is still required. One approach to reducing the size is to use a power divider along with a pair of transmission lines having different length for generating the 180-degree phase difference. Nevertheless, the size is still too large.
U.S. Pat. No. 6,661,306 disclosed a compact lumped element balun having a dual highpass and lowpass layout. As shown in FIG. 1, the balun 100 uses the overlapping lumped elements. Based on the band, the balun can adjust the capacitance and inductance, and use the metal wire and metal electrode on the substrate to obtain a plate and spiral structure having equivalent capacitance and inductance. The advantage of the design is that it uses a plurality of passive elements, and can even form a n circuit. In addition, the balun 100 is integrated with the filtering circuit to form a filter with unbalanced signal on one output end, and balanced signal on the other output end. However, the disadvantage of the design is that it requires a plurality of passive elements, and when a sensitive passive element has a slight deviation from its original design due to the manufacture process, the entire design will exhibit a different characteristics. In addition, the planar design also requires a larger space.
U.S. Pat. No. 6,483,415 disclosed a multi-layer LC resonance balun. As shown in FIG. 2A, the balun 200 uses a capacitor 201 and a plurality of coupled lines 202a–202d to form an LC structure, which has adjustable capacitor and transmission line on one side. In FIG. 2B, the balun 210 uses two or more capacitors 211–212 and a plurality of coupled lines 213–214 to form an equivalent LC structure. The unbalanced signal from output port is coupled to balanced signal, which is outputted from the balanced output port. The length of the coupling inductor and the area of the capacitor depend on the frequency band. The advantage of the balun is that it uses a stack structure to reduce the overall area of the circuit. The disadvantage of the embodiment in FIG. 2B is that it uses the scheme of half wavelength to obtain the ground equivalent to quarter wavelength to the coupled line center. This scheme restricts the reduction in the length of the coupled lines. In addition, the balun uses 6–8 metal layers; therefore, the effectiveness of the balun is very sensitive to the alignment precision of the multi-layer structure during the manufacturing process.
U.S. Pat. No. 5,497,137 disclosed a balun with a five-layer structure. As show in FIG. 2C, the balun 220 includes a first strip 222, a second strip line 226, and a third strip line 228. The first strip line further consists of a first portion 224a and a second portion 224b, which are coupled with the second strip line 226, and the third strip line 228, respectively. The disadvantage of the balun 220, as in the other conventional arts is that it uses the half wavelength scheme to obtain the ground equivalent to quarter wavelength to the coupled line center.